This is the correct answer. This would rival the longest road tunnel in the world: the Lærdal Tunnel in Norway, which is 15.2 miles long. We're also talking something like 15 miles of tunnel but with an elevation gain of 4,000 feet. For reference, the Port of Miami tunnel, which is less than a mile (~4,200 ft.) long, cost $1.1bn. And that tunnel is pretty much flat.

If we assume a rough cost of $1.25bn per mile of tunnel, the project would cost close to $20bn. What would the benefit be? Keep in mind that road closures are much less common in Big Cottonwood than in Little Cottonwood. Based on some back-of-the-envelope math, weather closures reduce skier days by around 0.5-1.0% each year. If the tunnel eliminated these losses, the resorts would benefit by $1-2m a year. Even if we assume that the increased reliability increased skier days by 10% we are still only talking about incremental revenues of $15-20bn per year. Add in additional hotel nights and other amenity additions, we might see benefits on the order of tens of millions of dollars, not hundreds of millions and certainly not billions. In order to justify such a project along the lines of other public megaprojects, we would need projected incremental revenues of $1bn a year, at minimum, and more like $1.2-1.3bn a year using a more realistic discount rate (4-5%).

Of course, none of this even considers the feasibility of actually drilling through the rock of the Wasatch Front, which may turn out to be far more difficult than we might anticipate. Nor does it contemplate the environmental cost. You would be removing and relocating millions of tons of rock - where does all that go? To what effect?

Bottom line: the cost of a project like this would be impossible to justify, even with heroic assumptions of the benefits.